If a planet does not make any Ptolemaic aspects then it is considered to be unaspected. It has an isolated, pure quality which lacks integration with the rest of the chart.
Whether it is radically different in quality from the rest of the chart will determine how much difficulty is experienced. For instance, if there is an unaspected Mercury but the chart has a strong third house then Mercury will not be so alien but an unaspected Moon in an earthy or airy chart with Mercury Saturn aspects may have real problems with accepting, recognising and processing emotions and needs.
Unaspected planets can be awakened by transits, progressions or synastry. Focus on the planet can bring it forward in the life and the experiences gained in that time period or with that person or organisation can help to integrate the principles represented by the planet into their experience.