I typically say yes, but it also depends on various factors.
Usually I recommend to individuals that SCOBYs can be stored in the refrigerator for up to a year, with increased chances of the SCOBY remaining viable the better your sanitation is. In addition, I usually do not recommend individuals to leave their brews out for more than a few weeks, but this is also dependent on several factors (for example, fermentation goes slower at cooler temperatures). If you do leave your brew forgotten on the counter, it's still possible that you can use your kombucha mushroom.
Typically, kombucha smells like hard cider or apple cider vinegar. The longer the kombucha and the mushroom are stored, the more acidic the kombucha will be in taste and smell. Here are some suggestions for what to do if your kombucha is too sour.
If you notice anything funky, such as mold, or off-odors, it may be safer to just discard your kombucha mushroom in the compost. If this is the case, you may consider growing a kombucha mushroom or finding other sources for a SCOBY.
However, if you don't find anything out of the ordinary, you may just want to give brewing with the mushroom a try! If you have some fresher starter tea to use as well, that can also be helpful.
If a SCOBY has remained dormant in the fridge for awhile, it may take a few cycles for the mushroom to kick back into gear. And if your intention is to grow a nice thick mushroom (because brewing good-tasting kombucha and growing a thick SCOBY do not always equate to the same thing), you may consider reading this post for tips. Once you have a good younger mushroom to brew with (or a few), you may want to put the older SCOBY to rest in the compost. In my experience, the younger mushrooms tend to out-perform the older ones.
Unsure if your brew or mushroom are safe? Read Happy Herbalist's Kombucha Cautions and Safe Brewing Tips. Some pictures of healthy mushrooms can be found in this post, and pictures of healthy (and unhealthy) mushrooms can be found on Happy Herbalist's Kombucha Pictures page.