I restarted my computer when troubleshooting an issue with another program and when it turned back on I noticed that RStudio had disappeared from my desktop and where it had been sitting on my dock, it had been replaced by a question mark.
I went on the RStudio website and downloaded the installer for Mac, but when I click on it I receive a warning "The following disk images couldn't be opened: Image: RStudio-1.0.153.dmg, Reason: No such file or directory".
I have the most up to date version of R and did have the most up to date version of RStudio — I had just downloaded them last week! Mac OS is High Sierra 10.13.6.
Please help! I need to use the program to do my stats for my thesis and I am kind of freaking out!
The file you download should be called RStudio-1.1.456.dmg.
Given your symptoms, I wonder if maybe you didn't actually install RStudio originally? On Mac, RStudio doesn't use an installer wizard. Instead, the app is distributed inside a "disk image" and must be copied to your computer via drag-and-drop.
When you open the .dmg file you downloaded, the disk image will mount. Think of it as being like if you connected a read-only thumb drive containing the app to your computer. You could run the app from that thumb drive, but the app wouldn’t have been copied to your computer. The next time you restarted, the thumb drive would be automatically disconnected and the app would be gone. The same thing happens with disk images.
Here are step by step instructions for how to install an app distributed this way: http://www.ofzenandcomputing.com/how-to-install-dmg-files-mac/
The appearance of folder and disk icons has changed somewhat since that post was written, but the basic principles still apply. You need to drag the RStudio app from the Disk Image window into your Applications folder in order to actually install it on your computer. Once you’ve done that, you can eject the Disk Image (just like if it was a thumb drive!) and delete the .dmg file.