I have a 1989 560 sel and the transmission stays in first gear and won't shift, can it be adjusted at low cost?

It was working just fine when I got to the gas station and turned it off to fill it up, then when I started it up and started going down the street it wouldn't shift to 2nd gear and hasn't been able to shift since then, is this a case that could turn into a major expense or is their a chance that it's not a major mechanical job and I won't need a rebuilt transmission?