I've done some Font changes, alignment adjustment for around 100 pages in FM 2019 and saved it. When i opened the document in another system in very next day, the changes are not get reflected? Kindly guide to solve the issue.
no other "accident" happened (like overwriting them with an older version),
you did not import formattings from some other (older) document,
my hunch here is, that you were doing these edits in XML files (like DITA), not in an unstructured or structured, binary .fm file. Is that possible? If so, you will need to make sure, that you do these changes in the template of your structured application, not in the XML files themselves, as the changes get lost as soon as you close the file. Alternatively, you can work with a developer to extend the structured application by adding special attributes to the elements you need to work on and then set the attribute values accordingly. E.g. you could add an attribute @align to an element <p> with values like left | center | right, and then define in the EDD to read this attribute value and assign the formatting to the paragraph accordingly.
I case you are talking about unstructured or structured, binary .fm files, probably one of the reasons mentioned at the beginning is the reason or some other reason that is outside of FrameMaker.