point-of-sale shutdown is effecting 8,000 starbucks locations in us and canada .
company said in statement glitch was caused by a failure during a daily system refresh .
glitch also affected starbucks ' evolution fresh and teavana stores .
many starbucks locations closed their doors so as not to give away free drinks .
coffeehouse chain said in update on its site stores are expected to open for ` business as usual ' saturday .