Logging In After the Ibook Has Been In Standby Mode



I have a problem with my ibook, only recently. When my ibook has been in standby mode for a while, and you want to use it again, it asks for my name and password. Unfortenately when I type those in, (and I tried this many times) my ibook says it's incorrect and I can't log in. Instead of logging in, I need to shut down my ibook. I also need to type in my password to be able to do this, however in this case my ibook finds my password correct. When I restart my ibook, I can log in perfectly again.

Could anyone please tell me how this is possible, because I don't understand why this keeps happening. I'd appreciate your help very much. Thank you.