How does limited time .SOF file work ?

I am new, and I created simple program that turns LED ON/OFF via one switch on board. This is done via NIOS II. I then created program in Eclipse to do that. I programmed it, and it all worked fine as expected.

Three days later, I am unable to even program...it fails. I re-generated the HW in Platform designer, then recompiled in Quartus Prime. Still unable to program the board. I noticed when launching Programmer from Quartus Prime, there is no Message Window to show why it fails, but if I launch it from Eclipse, I do see it, and it does say that there is time limited IP used, so I guess it might have expired, and that is the reason.

The only real HW I used is NIOS II. Maybe that is the IP it was referring to.

I really thought after programming the device, it will work for one hour, or until it is untethered, but it seems it is behaving differently...it seems it is a one-time limited ! If this is the case, I would say it is so unnecessary restriction, and hinders initial learning.

Anyway, I am hoping to get some explanation...

Thanks.