This a quick and dirty FAQ Troubleshooting guide in case is something isn't working for you.

Q: The game isn't saving my data!
A: Have you made sure to turn on "Enable Studio Access to API Services" under "Security"? If it's already on, then it's a issue on ROBLOX's part.

Q: The meshes and decals suddenly disappeared!
A: Roblox Studio probably updated recently, which means that they deleted all the data in the old version to free up space. Either that or you've just misplaced the "content" folder. If so, just copy the "content" folder again, and follow steps 1 to 2... you still have the "content" folder right? Don't forget to restart Roblox studio just in case. If the issue still persists, then it's a issue on ROBLOX's part. 

Q: I can't open the .rblx file!
A: It's either that the file got corrupted during downloading, or you don't have enough space on your computer. Just try downloading the .rar file again, and make sure nothing goes wrong during it. For the latter, clean up your computer, delete unnecessary files, and if you can't... then it sure sucks for you! Thanks for FloofyTheDraggo finding a solution for this.