If it's the before the start of the game (i.e. warm up mode) or if you are playing on a local server then the crosshairs are hidden. You can get them to appear by typing "cheats 1" in console to force the round to start or on a live server waiting for the warm up period to end.
If this is not the issue then try verifying your steam cache. Right click on ns2 in steam > properties > local files > Verify integrity of game cache
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If this is not the issue then try verifying your steam cache. Right click on ns2 in steam > properties > local files > Verify integrity of game cache