Sometimes, in the shared opensolr cloud, the data folder may get corrupted, so it can't be read from or written into.
One easy fix for this, is to simply remove your index, and then just create another one, preferably under another name.
If that doesn't work, please contact us, and we'll be happy to fix it up for you.
Also, keep in mind, that there may be more reasons, so please make sure to check your error log, by clicking the Error Log button inside your opensolr index control panel, and keep refreshing that page to make sure the errors you'll see are accurate.
If you do see errors in there, please email them to us, at firstname.lastname@example.org and we'll fix it for you.