Usually, if you slap the thermo switch when the car is cold and the power is off, the switch will reset. However, it may again stick in the "on" position when it gets hot. At that point, simply buy a new one as it isn't salvageable.

__________________
'76 240D-Sold
'78 240D-Sold
'85 300 SD, 165K-Sold
'88 300 TE, 165K-Sold
'64 Porsche 356C Cabriolet- under restoration
'86 560SL 124K Miles-Sold
'94 320E Wagon, 74K Miles-128K Miles JUNKED
'06 E350 Wagon, 84K Miles
07 SL550, 14K Miles