Rapi.dll is the Microsoft Corporation running process for Mobile Device Remote API. If your system detects its absence, here is what you can do. Firstly, if you remember that you have deleted the file by yourself, look for it in Recycle Bin or use a file recovering software, such as Recuva Data Recovery. Use the System Restore option to rollback your computer to a previous date(Start> All Programs> Accessories> System Tools> System Restore). The missing files might be caused by a virus or malware infection, so it is a good idea to fully scan your computer with an up-to-date antivirus. If the error appears when you use are using a certain program, reinstalling the application will restore the missing dll.