I've read enough self-help books (and by enough I mean too many) to know that there are many reasons, and procrastination is an often abused term to mean a lot of things including executive dysfunction, lack of motivation, or just tiredness. Note that laziness does not count here because there is no such thing as laziness really, it's another umbrella term for a variety of things like not wanting to do the thing, being too tired to do it, being shamed into thinking you can't do it (fear of failure), or just the general perception that someone else is not performing as well as you are imagining they should be. Some quick general tips that might help:

Write down the things you have to do. It can be in some tasks app, a text file, a notebook, anywhere, just make sure you don't spend too much time fussing over how you write the tasks and just write the tasks themselves, don't procrastinate on that! Then see if they have deadlines, which lets you sort by urgency, and also sort by importance, which is different from urgency (look up the Eisenhower matrix to learn more). Writing tasks down lets you get the thoughts out of your head, which might make you feel less overwhelmed, and lets you also find out that many things do not need your immediate attention or are not as important as they seem.

Split the tasks down into extremely small parts. Depending on how unmotivated you are, you might have to split it down to "put the laptop on the desk", then "turn the laptop on", then "type in password", then "open text editor" or something like that. Splitting like that makes the tasks feel so incredibly dumb or easy that you just can't turn them down, and if you only set as a goal to do those very small tasks instead of trying to do something very broad like "work on thesis" then you will get started more easily.

You can also get someone you trust to force you to do the thing, for example tell them that by the end of the day you will write a whole page in some document, and they will badger you into doing it, and maybe you'll feel bad enough to be doing it. Do that only with someone who wouldn't shame you for failing and always encourages you, or this can end up being really bad for your mental health.

Also make sure to go outside at least a little bit each day, even just to walk for a few minutes, get some sunlight and some fresh air. Drink water, get some rest, that can boost your motivation a bit. Rest is productive and necessary and not a waste of time.