Well this happened last night:
Was at work yesterday afternoon. My girlfriend Ellen sent me a text saying how her laptop isn't working and she just gets a message about the fan not working. Told her I'd have a look at it after work and give it a clean out.
It's a HP Pavilion G6 laptop, so fairly straightforward to disassemble. Got to the heatsink and fan and took it apart and was greeted with this when checking the fan out:
Over time the fan must have just been sucking dust in from various places and it's got caught up and just snowballed round and round until eventually it jammed the fan.
Cleaned it all up and gave everything a good blast with some compressed air and it's as good as new now. Runs much quieter and cooler now after putting some new thermal compound on.