[AndyK (Andy)]

I have a 2 x Barlow which came with my 8" dob when I bought it (used) and it has exactly the same problem.

Although I haven't had the chance to experiment, I suspect that the Barlow in my case is simply too long, and that the focuser doesn't have sufficient range to reach focus with the Barlow in place. I think maybe one of the "shorty" Barlows would be more suitable. In my case the construction of the Barlow lends itself to shortening the extension tube somewhat and if it proves to be overly long, that's what I'll try.

In your case I'm wondering if you have another eyepiece. The range of focuser movement either side of focus can be very small and can easily be missed at high magnification. Rather than trying at the high magnification of the 10mm, perhaps you have something like a 25mm or longer you can try?

If you can't find focus with any eyepiece, try undoing the screws holding the Barlow and Eyepiece in the focuser and slowly pull it out to see if you can then reach focus. If you can then you may be able to fix things with an extension tube.

Did the Barlow come with the telescope or was is an accessory you bought separately?