Gradient Fills converting to individual lines of colour

Hi. So I've seen this problem multiple times over the years and have always assumed it to be a version control issue however I wondered if anyone can shed any light on this - why do some files with gradient fills (which were created in illustrator originally) when reopened convert the gradient fill into a series of multiple lines of colour (no fill just stroke colour) within a clipping box? So you can no longer control the gradient as a fill and you end up potentially with a lot of lines which don't graduate as seamlessly as a fill... Anyone had this issue and/or know why it occurs? Anyone know if it's possible to reverse it once it's happend? Thanks in hopeful anticipation!