I've added some columns (name, ID etc ) to the document library after the documents have been uploaded, and got no value displayed.
Is there any way to fix this or I'll have to recreate the library?
Make sure you added the columns to all content types associated with the library.
After uploading there will be another window which appear where you can select the content type, and then enter the new metadata columns you created
The documents were already in the library prior to adding the columns? If so, the columns will not auto-populate that way. You will have to update those columns manually. Any document you upload going forward you will get the pop-up to select, input etc. information into those columns.