What happens if you owe money to inland revenue and cant afford to pay it straight away?

I'm really quite frightened as i have filled in tax return form, and though i had more than one job, i think i might owe hmrc money which i cannot afford to pay (like 5,6 thousand pounds) what happens do the put you in prison, will i need to sell my flat to pay of what i owe, please help as i cant sleep at night for worrying i cant afford to lose my job