disputed invoice and lack of communication

hi PPH

I have worked on the site for many years now and had alot of very positive experiences, I've never had any problems at all. However I took on a job in January - I did as I always do and kept buyer updated and worked with them - they were initally happy and responsive - hwoever then they became quiet and I heard nothing from them - I wasn't sure what had happened and I was getting no replies what so ever from the workstream, I decided to invoice for the deposit (which is only a third of the total price) and asked if they still required the rest of the job completeing there really is very little left to do - but can't fully finish without their feedback- I heard nothing but then got a 'invoice declined' notice and suddenly they responded saying they didn't know why an invoice had been raised . I can assure you that alot more work has gone into the job than the price of the depsoit- but anyway I then worked more on the job and have been messaging again in the hope they now continue to respond - but a week has passed and silence again - what do I do? I have worked on this job enough to merit the full price - but I assume that as they only have a small deposit that I'll not be able to recieve the full cost?