Payg phone??? Did you register it??? Yes you can, just call customer services and tell them you lost it, they will send you a new sim, may cost around a fiver for the replacement sim.
the good news is, if someone hasn't used it, you'll get your credit back too.
If you need a new handset too you could think about buying a new payg handset on that network, they may treat it as an upgrade and give you some free credit.
if you need the customer service number look on their website.