The disciples saw Jesus risen from the dead, though technically, not all of them. Thomas was not with the rest of the disciples, and when the rest came to tell Thomas about how Jesus rose from the dead, Thomas was very skeptical at first. After all, resurrections aren't normally a thing we see. Thomas said that he will not believe unless he see "the print of the nails" and "put [his] hand into His side" (20:25). Thomas is pretty specific on the evidence he wants to convince himself, though strong evidence does help ground people in beliefs.
But then Jesus himself came to Thomas. Jesus not only proclaimed peace to Thomas, but also told Thomas to do the very thing Thomas asked for. From the comment, Jesus told Thomas to "not be unbelieving, but believing" (20:27). Thomas did so, and he was amazed that Jesus's resurrection feat was legit! He proclaimed, "My Lord and my God!" (20:28)
Jesus proclaimed that Thomas believed because he saw and that there are blessings to those "who have not seen and yet have believed" (20:29). Today, I'm pretty sure there are many questions on Jesus's resurrection feat and its legitimacy. But while we lived thousands of years after Jesus's resurrection, there is still evidence that got great thinkers like detective James Wallace and writer C.S. Lewis to realize that what Jesus did was legit. For instance, since God can work outside of physics (the Big Bang itself implies that the universe is created and has an outside creator), feats like the resurrection are viable. The sudden transformation of the disciples (from fear), Thomas (from skepticism), and even Paul (a former killer of Christians) can also serve as evidence. The book Cold Case Christianity as well as sites like Cross Examined are great resources for those curious about the resurrection feat.