It is not very difficult to get back a missing block. The first step to reinstall a block that has been removed is to copy that block out of the [webroot]/concrete/blocks/ up into [webroot]/blocks/ (unless you've done an upgrade, check for that *)
So if you had removed the 'form' block, you are looking for the "forms" folder.
cp -r concrete/blocks/form/ blocks/
over SSH, usually from your public_html or www or whatever your hosting company named the public facing section of your account. Or you can do that on FTP, a cPanel-type file manager, etc...
Then you can go back to "Add Functionality" on your dashboard and you will see your missing block in the right-hand column. Once it's installed you can go ahead and actually remove the copy of the block in [web]/blocks/ That way if there is a change during an upgrade, your copied block won't override the existing one.
- Look in the folder called "updates." If you see anything in there, you will want to navigate through until you find the "concrete/blocks" folder. To date, most likely your site will have a "concrete18.104.22.168" folder in updates, you will want to copy THAT block down into the blocks folder in your webroot. That will be the most recent version of the block.