This could happen for two reasons. First, and the simplest — you accidentally pressed the run-toggle button (Q button).
You just have to click on it and your character will again run.
The second reason — your inventory has exceeded the maximum allowable total weight of items and (or) their number. Number of items, at which your character will go one step — 201.
Weight, at which there will be a noticeable slowdown, indicated at the bottom of the inventory window. Carry weight can be increased, number of carried items — no.
Use planetary storage to quickly solve the problem (storage terminals), to shift some of the items into it.