Most likely there was a "stuck" or "ghost" armory process (usually armoryDB) that wasn't being killed when you closed the Armory GUI down.
Best thing is to try and shut everything down, then restart your computer and try starting Bitcoin Core (let it fully sync) and then start Armory. That should enable it to work properly. If it doesn't, you'd need to post up your log files from Armory. (Don't try and copy/paste the contents of your Armory log files directly here... they'll be too big, so use
https://pastebin.com/)
Also, just checking that you are using Armory version 0.96.5 from here:
https://btcarmory.com/ (DO NOT use bitcoinarmory.com!)
and Bitcoin Core 0.19.0.1?