1994 GMC Safari

Hi dear,

I just try turning on my heater on a 38 degree celsius freezing day ... and neither the heater or the cooling fan is working ... but I was using this van since this morning and it was working fine the whole day until now ... now there is No heat nor cold air coming out from the vent when I tried turning it on ... What has happened and what has caused it ? Please help!!

Thanks in Advance ^_^
Izu