2001 Golf with shifting issue

It is a 2001 1.8T Golf, automatic. 75000 miles on it, mostly highway. I returned from my recent road trip, halfway across the country, which involved a couple of snow storms but otherwise everything ran smoothly.
The day after I got back, I was going to dinner and the car wouldn’t shift up out of 1st gear… unless I gunned it to almost 40 mph. Next time I started it, the check engine light turned on. I have driven it 3 miles, twice, since then and the problem remains.
Any ideas on what the problem might be, if I’ll need to replace transmission or just some fluid, or get a new car?
My appointment with the DEALER is tomorrow morning…

We would need to know the trouble code(s) stored in the computer to make an educated guess.
If the transmission fluid hasn’t been changed in more than 30k miles that would be a good thing to do regardless.