HPV can be inactive (latent) so it is possible that you could have had it at your last smear test (it can't be detected when it is inactive) or you could have picked it up since then.
HPV testing in the UK cervical screening programme is relatively new, but before it was widely introduced, some parts of the country were testing for it in pilot studies. Unless you were definitely told that you were HPV negative, it is possible that you weren't tested for it when you had your last smear.
How anyone picked up HPV is always a mystery, but it is a very common virus that a lot of sexually active people come into contact with. Most of the time it causes no harm because our immune systems get on top of it. But with cervical cancer, if HPV persists, then over time abnormal cells can develop which might eventually over more time lead to cancer. But this is where cervical screening comes into the picture. The idea is to pick up abnormal cells, assess them and when needed, treat (destroy or remove) to prevent cancer from developing.
I expect you will be called back for another smear test in a year. This gives your immune system more time to clear the virus and if it doesn't and any cell changes do develop - which may not happen - they can can be assessed and if needed, treated then.
A positive HPV test usually causes a fair bit of stress and triggers a lot of questions. It might be worth looking some information including Q&As on the website of another charity, Jo's trust.
