serial port busy means you have some program open (probably a monitor or terminal of some sort) when you tried to upload, close that program.
What is an Uno Plus? Can you link to the documentation for how it differs from a normal Uno?
Does the LED blink a few times when you try to upload? If not, that implies that either the bootloader has broken itself (highly unlikely) or that reset is disabled (maybe one of the features that makes this board different from an Uno is a jumper to enable/disable autoreset, and you used that last time?)