Is it one of these?
https://www.amazon.co.uk/Aqua-Dri-Portable-Dehumidifier-Moisture/dp/B0015M2492
https://www.amazon.co.uk/Aqua-Dri-Portable-Dehumidifier-Moisture/dp/B002T3VPCS
Assuming the power supply is OK (if the fan is running, that would seem to be the case) there is not much to go wrong, other than probably a failed Peltier element.
Best bet seems to be to simply buy a new one - unless yours is less than 2 years old, in which case the 2 year warranty may be useful.
XXX, November 2016