Air conditioning unit will not turn on.. blower works but not the air unit. Inside the heat pump in basement , when we 'jumped' the two wires for the low pressure switch it engaged the unit to work for that temporary moment. What are the reasons for this? Low freon, bad pressure switch, other? Nothing looks frozen or leaking.

Most likely low on charge, switches do go bad. But not often. Does it have a reset on the switch??

I don't see a reset but it looks now like there may be a leak.. rusted down side of what looks to be a round tank and some rust on floor.