Unable to access a catalog for Lightroom Classic(desktop)

[Cindyj1]

After taking an update from Adobe I am no longer able to access my catalog and had to create a new one. When I try to merge the old with new- I receive an error message indicating that catalog is "in use" by another application. I've checked if another instance of LR was operating- no - shutdown all applications- on reboot still receive same error message. How can I access my 65k photos on my previous catalog and merge into the new one. 

[Peru Bob]

I've moved this from the poorly named Community Help forum (which is the forum for issues using the forums) to the Lightroom Classic forum so that proper help can be offered.

[dj_paige]

Catalog "in use" by another application ... find the preview cache, it's in the same folder as the catalog file, it is a subfolder that has a name that ends with previews.lrdata (might have some uppercase letters), and delete this previews.lrdata sub-folder ONLY and DO NOT DELETE ANYTHING ELSE.

[Rob_Cullen]

More than likely the catalog "in use" was not closed correctly and a 'Lock' file remains to prevent it re-opening.

See the instructions here to delete the 'Lock' file and re-establish the 'old' catalog,

LOCK FILE DELETE

The same warning from dj_paige applies! DO NOT DELETE ANYTHING ELSE.