The first thing to try is on this computer go to Timeslips administrator > Diagnose this computer. You may see a red x under firewall. Click to open port 3050. For some reason I find I often have to click twice. Once it is all green. Close and try Timeslips again. 

Using the timeslips diagnostics I found that the firebird service was not started. I clicked on it and it fixed the problem.

Using the timeslips diagnostics I found that the firebird service was not started. I clicked on it and it fixed the problem.

If this recurs, it would be a good idea to have an IT professional track down whatever program or issue is stopping that service from running.  That is not normal behavior for it to be stopped for any reason except manually.

If 3rd party malware software is stopping unknown (to it, anyway) services, it should be told to disregard Firebird as it needs to be running for Timeslips to function.