NES Blinking/Flashing fixes
A very common problem with the front loading Nintendo Entertainment System is it failing to recognise that you have inserted a game. The image on the screen flashes grey, whilst the power led blinks red.
If it's just one game then the chances are that just that cart needs a clean. If you have a Gamebit then it's probably easier to take the cart apart and clean the contacts (mmmonkey just rubs a pencil eraser across them) then put it back together and test it. Of course you may be able to get the official NES cleaning kit which includes a cartridge cleaner as well as a cleaner for the cart connector.
If that didn't work, or if it's a problem with any game then it is probably a problem with the cartridge connector in the console. You may find that holding the game cartridge down fully when you switch the console on makes it work. You should try cleaning everything related to the cartridge connector, taking the console apart is easy, once you have removed the motherboard you'll find that if you pull at it the cartridge connector will slide off the motherboard.
Once removed, check for any obvious signs of dirt, remove them using something thin like a blade/small screwdriver. If you can, clean the contacts of both the connector and motherboard and then partly reassemble your console and test it.
If that fails to work you could try bending the contacts of the connector, mmmonkey uses a small screwdriver to gently lift each individual contact up. It takes a long time but it can sometimes work. Look at the photo below and notice how the contacts on the left stick up further than the others.
Again, partially reassemble the console and test it again. Still having problems? Then you could be very patient and remove the connector again, this time with the intention of bending a difficult to reach row of connectors. Use a screwdriver to twist and lever out each of the contacts in the first picture below. Using a pair of small long nosed pliers you can bend the contacts to a similar shape of the one in the second picture below (the contact on the left is the original shape, the other has been bent).
Complete this for all the contacts (put a CD on to stop you going insane whilst completing this!). If this still doesn't fix your problem, and your not in a rush then you could now try the same as mmmonkey. Normally you shouldn't leave cartridges in any console, but if you've done all the above then you could try leaving a game in for a couple of days, then trying again.
The fix which is virtually guaranteed to work is to simply buy a new cartridge connector for your console. eBay is your friend when looking for this, simply search for NES Connector, if that didn't work change the search criteria to search worldwide, there are usually some sellers in America who send them worldwide if needed. You may also be able to buy one from here or here (mmmonkey has never used either supplier before).
Some people also report that disabling the lockout chip can help fix this problem, it certainly stops the flashing screen and blinking light, but you might still end up with just a grey screen.
Once you have fixed it one way or the other, NEVER leave a game in the console.
This procedure is very well documented all over the web, mmmonkey first saw it on Mark K's website, and recommends that you visit it to read a more technical document on this procedure and other mods.
