it could be that the server was down at the point of you trying to connect, that you have an out of date client version, or on a rarer occasion, your isp, whether it be a company, or say a university if you're in a dorm, has disabled the ports needed to be open to let you connect to PS.
We'll start with the easy two to troubleshoot.
Have you tried opening the client since then? If so, has it shown that the server is up or not?
what is the client version it states you are running? I believe on windowed mode, this is at the top bar area.