2004 Citroen C2 Citoen C2 engine audo diagnosis light

Engine Performance problem
2004 Citroen C2 Manual 41600 miles

Hi there,
Have been driving my citoen c2 car today and on the way home the Warning light that represented the engine auto diagnosis came on the panel, then the car slowed and juddered, I was lucky enough to be only a few houses away from my house and got home ok, I turned engine off anf started car agen and the exhast made a noise and engine light came on again, any ideas wot the problem is before I take it to a garage and if so how much it wud be to repair :( thanks

kind regards amanda