one thing that just came to mind ... check the DVD itself. Is there dirt or smudges on it? A dirty disc can create problems that were not present on your last attempt using it.
If so, take a slightly wet tissue and gently wipe it clean (NEVER do it circularly, that's no good). Maybe that can get the pc to get the disc working as it did before.
For "bootmgr missing hit CTRL + ALT + DEL to restart the system" error, see the instructions
listed on this site, I think one of those methods should take care of the issue.
Also, I am not sure what that error 121 is about, unless you can provide more info on what specifically the error message said and also I assume your Toshiba PC is in good condition (barring the operating system issue)
Good luck