Hi, I'm new to the forum and hoping to get a bit of advice re: my 2006 TD4

[Ally31]

Anyone out there have any idea what might be wrong with my TD4 (2006) ... took it into garage a couple of days ago to get the brake light switch replaced, and when I went through to pick it up, it really struggled to start up. It's been parked up over the weekend, but when I went out to try it a couple of hours back, all I got was the engine turning over - but it absolutely WILL NOT START! The mobile mechanic's had a quick try, and he's pretty sure it's not the battery - but seems as puzzled as me at this point as to what the cause might be. In almost 2 years, it has NEVER failed to start up, so am planning to call out the AA tomorrow - but, for now, I'd REALLY APPRECIATE any help/advice from anyone?? Thanks