PhD dissertation burnout duing pandemic (COVID-19)
From what you've said you have already slipped into depression precipitated by external factors associated with the COVID pandemic. You could screen for this by taking a PHQ-9 questionnaire which is probably what a GP would do (https://www.mdcalc.com/phq-9-patient-health-questionnaire-9) but if you are crying daily and cannot concentrate then it probably isn't necessary. 

If you have had thoughts of suicide or self harm you need to seek urgent medical help. If not, you still need to seek professional help on a semi-urgent basis. You should apply to your university's disability services or whatever it is called at your university for special circumstances and adjustments for which you will need a letter from your health provider and you should do this early rather than at the last minute. 

In the meantime, you should try online cognitive behavioural therapy websites such as Headspace (https://headspace.org.au/) or Unmind (https://home.unmind.com/) or whichever equivalent your institution may have access to.  Some have made their platforms available for free during the COVID pandemic to certain groups and this may be worth a look. You need to tell your supervisor and consider pausing the PhD or going part time if you can. 

Most importantly, you are not alone and you can find many people who have written about how the COVID crisis has negatively impacted their mental health (https://www.theguardian.com/news/audio/2020/apr/24/what-is-the-covid-19-crisis-doing-to-our-mental-health-podcast) or (https://www.theguardian.com/world/2020/mar/22/think-about-the-best-case-scenario-how-to-manage-coronavirus-anxiety). 

Don't suffer alone, get help early, know there are many people who empathise with you.