This is a massive cliche but it's true: it's probably more scared of you than you are of it
I witnessed a pretty sizable eight-legged friend around my room last night, went to get them with the glass and when I came back it was gone. It could be anywhere. I was an arachnophonbe for years and am still overcoming the fear. But I thought, what's the worst that could happen?
I'm assuming you live in UK? In which case, you can relax. We have no venomous spiders here and to be honest, what we call big is laughable. (Look up trapdoor spider if you don't believe me, at your own peril :P)
Most critters are harmless until provoked, they just want somewhere to live while it's too hot, cold, dry or wet outside. Leave it be, it'll probably be gone by now
I hope this can help, just wanted to say you're not alone but there's nothing to be afraid of