I changed the oil and filter in my 97 F250 powerstroke and two days later while driving to work the check engine light came on and I lost some power. I could only go about 45mph. Any ideas?

You need to read the engine codes, you can take to a shop or buy a code reader for maybe $40.00

Check for codes. When the Variable Speed Sensor (VSS) goes out, your dash can light up like a Christmas tree and the truck will only hobble back home. Many codes may be thrown. After replacing the sensor, if you suspect this is the problem, clear all codes--PCM and ABS.