Loss of trust within a relationship may come about from many scenarios but perceived or actual adultery or unfaithfulness are probably the greatest causes. The important thing to realise here is that even if one partner does not see their actions as being unfaithful, if the other one does, then damage has already been done. Every couple has a different definition of unfaithfulness but it can encompass every possibility from a physically sexual affair to on-line pornography, it just depends upon the couple and their agreed relationship terms.
These issues do not mend themselves! Once trust has been lost in a relationship you need help to find ways to rebuild it so that it can again to become the foundation block of your healthy relationship. To reach this point, you will both have to make agreed changes in your relationship and be prepared to move outside of your comfort zones.
Where loss of trust arises from other issues such as financial instability or addictive behaviours that have threatened the relationship, these can also be worked with in therapy to find effective changes of behaviour.
However, couples who successfully work through loss of trust issues, frequently find themselves in a stronger and more enduring relationship with a deeper understanding of each other and appreciation of our human frailties.