There is probably an IP address conflict somewhere in your home network.
Turn everything right off - router/PCs/printer and any other connected devices such as TV or powerline adaptors.
Count ten (slowly) and then turn things on in the order :
Router
(then wait until all lights are showing for an internet connection)
The 'good' PC
(then wait until you have a working internet connection)
The 'bad' PC
(then wait until you have a working internet connection)
then everything else .. one at a time, checking all internet connections after each piece of kit.