With the lack of detail and symptoms, I have to guess it is staying in second gear. That is "limp mode" that allows you to drive it slowly to a repair shop. They will start by reading the stored diagnostic fault codes. As a general rule, if it starts out right away in second gear, it is typically a wiring or sensor-related problem that has been detected by the computer. If it starts out in first gear like normal, then bangs back to second gear during or right after an up-shift, and stays there until you turn off the ignition switch, it is usually due to slippage in the clutch packs indicating the need for a rebuild. Your mechanic can also view the "clutch volume index", (CVI). That is a set of four numbers indicating how much fluid it takes to apply each clutch pack. With that he can determine how much life is left in those clutches.