E46 Running Cold

[Zmass]

Anyone any ideas why my e46 325i would suddenly continually run cold? It's been left standing for a week in an open carpark and now the temp gauge won't go above one third, it was fine last week and the gauge always settled dead centre. There's no discernable difference to the drive only the heater isn't as hot.

Help please! Cheers

[Zmass]

Thanks guys, much appreciated. I can take it to the local garage now and tell them what I think they need to fix - it's a nicer position to be in than asking them what's up.