I recently accidentally clicked on a dialogue box that popped up stating something about blocking downloads. It happened so quickly I couldn't really read it. However, since this I am unable to open any of my onedrive photos in lightroom classic (I have them on onedrive to save hard drive space). All the onedrive photos have a black circle with an exclamation mark in. They won't open in "develop" or "Library". I beleive it to be a syncing issue but I cannot think how to remedy.
Black circle with an exclamation mark indicates a corrupted file, and it can't be used. You need to restore the file from a backup, which hopefully is uncorrupted.
Many people have problems storing photos in "cloud" folders such as OneDrive or DropBox, photos can get corrupted easily. So the advice is to not store photos or catalog in "cloud" folders.
Oh ok, thank you for the swift reply. Is it possible that it is unable to access becasue I blocked access to it?
No. The file is corrupted. Blocking access may have other affects, but the file is corrupted and can't be used.
LrC needs photos to be stored locally, they cannot be in the cloud.
I don't use Onedrive, but maybe the dialog you mention was about deleting your local copies on Onedrive to save space.
There have been several reports about Onedrive not working well with LrC, even with photos being available locally.
I use Dropbox, which works fine, as long as Save hard drive space automatically is turned off in preferences. When turned on, it will delete local copies.
I would buy an external USB hard drive, copy all of your photos from OneDrive to the drive (OneDrive is fine for a backup) and then see if Lightroom works ok with the photos on the USB drive.