I accidentally cancelled my subscription to discovery+,how do I get it back without being charged? I already purchased it for the month
99% of the time, subscriptions will continue until your due date. You should be fine until then.
That said, I don't know about what happens with 3rd party billing if you are subscribing through Roku. Yet another reason to always subscribe DIRECTLY with streaming services through their web sites instead of through a 3rd party like Roku, or even using a phone/tablet app (Google/Apple get 30%.)
@Fiend1380, in addition to what @andyross said, if you did get your subscription through Roku you should be able start it again by going to https://my.roku.com and clicking the "Manage your subscriptions" link. If it doesn't show up there then you didn't get it through Roku and you'll have to contact Discovery.