I purchased an N64 about 4 months ago and I tested a dozen games on it when I got it and everything worked fine. I stored it away in its original box and haven't touched it since. I pulled it out yesterday and tried to test a new game I purchased and there was no output at all, no audio or video. 

Here are things I made sure of: 
My TV works fine, the input I used works fine (tested my SNES on it) 
Cables are fine (tested cables with SNES) 
System is getting power (LED light is red) 
Tried games that previously worked 
Made sure expansion pack was seated properly and even tried the regular 4MB expansion pack 
When I first got it, I cleaned cartridges, cleaned system contacts, cleaned expansion pack and expansion slot contacts (everything worked fine then) 
Unplugged/waited an hour, replugged and tried it 
System's guts were cleaned from when I previously opend it up, doubt the insides are a problem 

No matter what I try, I get the same message from my TV: no input detected. I did some Goolging and found that the no audio and/or video is likely due to a loose video connection in the back, which I feel like is the problem as the AV cable does wiggle quite a bit compared to my SNES (have no other N64 to compare to). 

Is there a way to fix this? My searchs for fixing this issue haven't really resulted in anything. I found one that showed the problem being fixed by using Vaseline on output slot and the video cable plug... not sure I wanna try that or if it's even a legit solution. 

Get a Gamebit, a Soldering Iron, some 40/60 lead solder and reconnect it!

also quickly check to make sure the "reset" button is not stuck...ive seen several n64 get traded in this way.

dirt, dust and grim can build up and cause the the button to stick in the down position leaving the screen black. hope you figure it out....let us know.

I'm a bit lost by what I should reconnect? Nothing is detached, as far as I can tell...  Never thought of that, but it appears to be fine. I opened the system up and was able to press down on the reset button no problem. 

Got any pictures/video?

No. When it comes to the N64, the console will not initialize the audio/video output at all until it finds a program to boot. In this case, a game. The first thing to do is ensure the cartridge slot and the RAM expansion are clean. I guess you've already done that. Try this anyway, wet a cartridge and RAM expansion in rubbing alcohol and use those to scrub their slots. Wipe dry the cart connectors to remove the dirt and keep going until the carts are clean.

Change your jumper pack! I had the exact issue a few months ago. The system has been working perfectly, and then I went to play a game and got nothing. Tried swapping power supplies, video cables, everything. Figured I would try a different jumper pack and boom, system boots up.

If you have a newer tv you might need an rf modulator. AV cable input to coaxial cable output. Some new tvs dont always pick up the signal of the n64.

I'll try to reply in order...

I've been a bit tied up this weekend but will try to take pics of the insides next weekend.

Didn't realize the N64 doesn't boot if it doesn't pick up a game. I'll try re-cleaning all game/system contacts.

Both the regular and expansion packs worked just months ago... hard to believe they would both fail in that span but I guess that's possible. I'll leave this until the end as it means I'll need to buy another one.

The N64 worked fine on the same TV just a few months ago, don't see why it would cause problems. I'll try a different TV and see if that helps.

try cleaning the expansion/jumper pak with alcohal and a q tip. i had this issue and that fixed it for me. other than that im not sure

Only one of the two cartridge slots has to fail in order for the N64 to not work. Yeah, unlike other consoles, the N64 doesn't even show a black screen if a game or the RAM pack doesn't work. It displays nothing at all, as if it wasn't plugged into the TV.

Most likely the game cartridge slot has gotten dusty, but make sure all your games are clean too. Do you have another N64 to test your stuff against?

Dang, didn't know that. So there's basically no easy way to see if it's the expansion pack, the expansion pack slot, cart, or cart slot... gonna take a long time to test everything.:-/ 

I unfortunately don't have any other N64s... my brother has one but it's back at my parents place, which isn't exactly close.

Alright, so I think it's fixed!

I re-cleaned all of my games and tried them, no luck. Then I re-cleaned both the cart and expansion slots and everything works again! I wish I had done one at a time so I knew which one it was but oh well, just glad everything works again! Appears that either one of the slot contacts were too dirty... wish there was an easier way to clean them, not sure I got everything... might look into a cleaning bit or something.

Thanks for the tip Guntz, had I not known that about the N64 not outputting video if somethings not right, I probably would have just taken it in to get repaired.

Thanks everyone!