Programs do not just "disappear" for no reason. Reasons to look for: Virus, Hacker, "Little Prying Fingers".

You will need to check for a virus. Run a complete scan of your system. If you do not already have it, may I suggest MalwareBytes Anti-malware (a free download). In fact, use several anti-virus programs to check, as any one program does not catch everything. Once you are "clear" on the virus issue, it is probably safe to reinstall Nero. Yes, you will need a disc.

Change passwords.

Be sure that no one has access to your machine; such as a Hacker, Child, or some one that is mad at you and pulling a prank. It is possible that some one deliberately or by accident removed your program. Keep your machine physically and electronically secure.

Good luck.