You enter =7+8 and the cell still shows =7+8, not 15. First check whether many formula cells behave this way or just this one. That observation helps separate a display setting from an entry Excel has stored as text.
Many formulas are visible
In desktop Excel, open the Formulas tab and find Show Formulas. It switches between the expressions and their results. Toggle it and inspect a cell whose answer you already know. Seeing an expression in the formula bar when you select a cell is normal; the question is what appears inside the worksheet cell.
One cell stays as text
Save a copy of the workbook. Inspect the affected cell’s number format. If it is Text, change that cell to General, then edit and confirm the entry again. In desktop Windows Excel, F2 followed by Enter is the documented route; use cell editing when your keyboard assigns those keys differently.
Changing the format alone may leave the existing text unprocessed. Re-enter the formula in one copied cell first. For our arithmetic example, the expected result is 15. This test does not establish that an unrelated business formula is correct.
Avoid changing every text column
An identifier such as 00125 may intentionally be text. Converting the entire sheet to General can change data you meant to preserve. Limit the repair to cells that should calculate, and compare the copied workbook with the original before saving over anything.
If the cell already displays a number but fails to update, investigate calculation settings instead. Displaying formula text and retaining an old calculated result are different symptoms. These steps describe desktop Excel; web and mobile controls can differ.