Minecraft wiki entry for Tips_and_tricks, associated text: Baiting a ghast into blasting the empty portal again will work, as ghasts can light nether portals just as destroying nether portals.
Nether fortress and other structures may have chests with flint-and-steel or fire charges to relight the portal. They can also provide obsidian to build a new one.
The makings of a fire charge can also be harvested in the Nether and Nether Fortresses, if the chests are not being helpful: A ghast drops gunpowder, A blaze provides a blaze rod, and a wither skeleton drops coal, which all three can be combined without a crafting table to make a fire charge.
Gold can be mined in the Nether, and then traded to Piglins to get a fire charge and perhaps obsidian. They can alternatively give you enough iron nuggets to make a flint-and-steel (gravel is common in the Nether, especially in the nether wastes and soul sand valley biomes). Nether Gold Ore needs at least a stone pickaxe, but you also make stone tools from blackstone, which you can find in the Nether in bastions and many low Y-axis areas.
If you kill yourself in some sheltered spot, your items will drop and you will respawn in the Overworld. In single player, time won't pass (much) in the Nether while you're in the overworld, so you can more-or-less take your time about getting new equipment, then go back to quickly fetch your stuff. Naturally, you'd like to do this someplace where your items won't be burned up or stolen by monsters.
Pro Tip: Don't get stuck in the Nether. It's not fun since a lot of hostile mobs spawn there.