Western Master loco suddenly very slow

[TomWFox]

I recently purchased a Western Master E-link train set. At first it worked great, however one day I started up the loco and it would only go very slowly, setting it to the cruising speed did nothing, it wasn't even at its shunting speed.

This happened without warning so I am very unsure of the cause.

Any help appreciated, I'm new to model railways so go easy on me :)