Some technical issues can be solved by reinstalling Roblox. Make sure to try restarting before reinstalling. Here's how to reinstall:

Windows Users:

Reinstalling Roblox

The reinstallation should overwrite and replace any deleted or broken file. Make sure that all other programs are closed when reinstalling.

Uninstalling Roblox

Most of the time, you don't need to uninstall the program before reinstalling it. If reinstallation does not work, you may need to uninstall before reinstalling. Follow the instructions to uninstall Roblox for Windows. Make sure to restart your computer after uninstalling and before reinstalling.

Delete your temporary Internet files

An important step in any uninstallation or reinstallation process is to delete your browser's temporary Internet files.This will help with any corruption issues that may have arisen. Follow the instructions in the Delete your temporary Internet files and cookies section. Repeat the reinstallation instructions above when finished.

Change your firewall/security program settings

Many antivirus or Internet security software try to control which programs can access the Internet. These programs do not work with caution and by default no programs are authorized to access. See the information in the Firewall and Router Issues help article for assistance on allowing access to Roblox. Repeat the reinstallation steps after reconfiguring the firewall.

Mac Users:

Uninstalling

Make sure you're completely out of the Roblox program. Follow the instructions to uninstall Roblox for Mac.

Reinstalling

Open Safari, log into Roblox and play. You will be asked to install the application. Please do this by following all the instructions on the screen. If the installation does not happen automatically, do the following:

Go to the Finder > Today and click on Roblox.dmg - this should run the installer for you and place the Roblox icon in your apps.
Select it from your apps and it should go to your tray