LrC crashing since getting a new monitor

hmm, I ws not going to ask this, but... QHD vs FHD despite same diagonal size. Could be a large increase in resolution. Could be a large increase in GPU requirements. Remember that LrC is a huge resource hog that can and often does reveal hardware issues that d not crop up in other programs.

Please post your System Information as Lightroom Classic (LrC) reports it. In LrC click on Help, then System Info, then Copy. Paste that information into a reply. Please present all information from first line down to and including Plug-in Info. Info after Plug-in info can be cut out as that is just so much dead space to us non-Techs and it takes up vast amounts of scroll space making the reply less readable and less likely that others will bother with your post.

And what specific make/brand/model is that new monitor?

Do these two monitors, whatever brand, make, model they are use the same cable, the same port?

You would have disconnected at least one end of that cable, and the other end could have come loose, have you checked both ends?