It is possible that you have been hacked indeed.. IF you did not password (PSK) protect your network do so in the wi-fi security settings
If you did, Change it
You said you have been getting messages... Are these pop-up type with a "Click here to fix" link? IF so DO NOT CLICK ON THAT LINK.
If that is the case odds are good, so long as nobody has clicked, your system is secure and nobody is downloading spongebob or anything else you do not know about.. The message is a teaser to get you to install their malware so they can infect your system. Block it if you can.. (I can but am not comfortable telling you how since you can seriously mess up your machine).