As simple as going to the (text) console: just switch back to the (graphical) original console the same way you came. Try pressing ALT+F7, a lot of systems use that for the one where X sits. On some others it's F5 or F6 instead of F7, but try and you'll see; everything can be configured as pleased, but it's common that F1...F5 are text login consoles and from F6 onwards reserved for graphical ones, typically only one of them being "in use" (only one instance of X running).
Note that to get from the text console back to X you'll need to press ALT+Fn, n being a number, but when going from X to the text console you also need to hold CTRL (so it's CTRL+ALT+Fn; your ALT+Fn are usually reserved for something else in your graphical desktop).
Hope it helps.
Tried startx but it pops up a dialogue box "I've detected a panel already running and will now exit".
This is true; switching to the text console does not kill X or anything, you simply go from one terminal to another, leaving X still running. So trying to restart X either fails after it detects one is already running, or if you explicitly make it start, you get another X instance running aside the existing one. All this may be handy, but in most cases can be achieved by other means as well; if you do use the text console terminals, remember to log off them when you're finished, so you don't accidentally leave them open---if you just switch back to X without logging off from the text console, your login is left open for anyone who switches back to that terminal
. And if you're for example installing a graphics driver that says you need to shut X down before proceeding, don't just switch to a text console this way and rush forward, because the X isn't shut down, and so it may cause trouble
To shut X down, use CTRL+ALT+BACKSPACE, or if it keeps restarting right after you do it, use the init scripts to shut it down, or change runlevel to one where X is not running (using the (tel)init command).