When I hide a block there is no way I can't unhide it again, I don't know why, I think is a problem with the theme. The only way I managed to unhide it is opening developments tools from Chrome and then deleting the class hidden from the block.
Moodle version 2.9.2
Hi Vladimir
You can try to anther way to unhide block, go to Site administration--> plugins --> Blocks--> Manage blocks--> eye icon block HIde/Show
thanks