The only malicious thing that somebody could do with your router would be to disable to wifi password. Even then, it would only let others 'piggyback' your connection; it still wouldn't let them access you computer (or other devices), nor would it let them intercept data passing through your router. (I'm assuming that you've not got some very unusual, and incredibly lax, settings on your internet-connected devices).
Simply check that there's still a password on your wifi connection and all should be OK. If you're still worried, change the password. (If you need instructions, please tell us the make and model of your router).