Lightroom classic continually crashing either on start-up or within minutes thereafter.

I have been using Lightroom Classic successfully for the past 12 months however over the past week it continuslly crashes on start-up or with a couple of minutes there-after,  Initially had 28,000 photos but  have edited this down to 21,500. 

 I an using an IMac with MacOS Catalina Version 10.15.4

On opening today I now find that 700 photos are 'missing'.  Cannot now log on. HELP!