An easy first step is to try changing channels on your wireless. If everything has been fine up until now and you haven't made any changes, it could be just plain old interference.
If you can take a look and see what channels are being used around you that would help. Most people let the wireless router use the defaults. If you've done that as well there's probably a boatload of people around you on the same channel and that can and will cause the kind of problem you have.
If that doesn't work we can dig a bit deeper.