All NPC Character sheet stats gone

Got an issue that has only started. During the session I tried to open the character sheet for an NPC and it came up with a blank sheet and the pop up asking me how I wanted to create the character. I tried going directly using the token in the Journal tab and got the same thing. I have just checked and this seems to be the case with all the NPC's...only the Player characters seem to be unaffected. I have tried rolling back the game to a couple of days ago but it's still the same. Does anyone know what may be causing this please?

I saw something similar a while back on the Pathfinder by Roll20 sheet, though it was affecting me as a player.  What fixed it for me at that time was logging out of the game, clearing cache, and then logging back in.  Not sure if that will solve your issue, but might be worth a shot if you haven't cleared  cache recently.

I suggest contacting the Devs via the Help Center as the forums are primarily users helping users.  With that said, what might have happened is the setting for the PC/NPC switch got corrupted switching all NPCs to PCs.  Please click the "cog" icon next to the Core/Bio/Spells buttons, then please post a screenshot of the top right corner where it says Character Sheet Type.  Next, if it says NPC but the sheet is showing PC, switch it to PC, then back to NPC. Alternately, if it is showing PC please switch it to NPC. See if the NPC data is still present.  I am hoping this was the issue and not something more serious. Either way, the Devs will need to be informed. I have been seeing an increasing issue with the switches on the 5e by Roll20 sheet. 

Thank you both for your replies. I have to admit that this is my first post on these forums as, in almost two Years of use, I have never had an issue with R20 (I do use the basic core functionality though) until this. I just logged in to try what you suggested Gauss but it, strangely, seems to have fixed itself? I was trying everything I could think of yesterday (including such things as clearing cache as Kaynic suggested) with no luck....but now all NPC sheets are opening correctly and will full stats. I will keep an eye on it (I need to add a couple of maps and some new NPC's this weekend before a session on Monday) and, if I get the problem again, will contact the Help Center. Thank you both for taking the time to try and help with this though, really appreciate it.

If it does happen again please let me know, I'd really like to know what is happening here. :)