No power when trying to accelerate but revs go up - any ideas?

As I went to drive home from work my rover 25 was very slow whenever I tried to pull. I thought I had accidentally gone in to the wrong gear but I hadn't. I don't seem to be able to build speed up above 20mph. My revs jump up to r/5 but just have no power. Any suggestions as to what the problem could be?

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